It is known to provide acoustic damping liner assemblies in casings for turbomachines, such as gas turbine engines, to reduce noise generated by the engine during operation. Typical assemblies consist of one or more acoustically resistive layers arranged radially inward (relative to the axis of rotation of the fan of the engine) of one or more acoustic damping honeycomb cavities. These liners are tuned to attenuate noise within a range of frequencies.
Typically, such acoustic liners are provided forward of the fan (i.e. upstream relative to the flow of the gas through the engine). The acoustic liners are typically provided as a circumferential assembly arranged around the inner surface of the fan casing. Each acoustic liner is typically referred to as a front acoustic panel or forward acoustic panel (FAP).
Conventionally, a FAP is made up of multiple components and materials, as shown in FIG. 1. For example, two partially circumferential compression moulded plastic rails 1, 1′ arranged front and back (i.e. axially spaced apart) provide structural rigidity and a land for the mounting arrangement. The core 2 is formed aluminium honeycomb, bonded in, with drainage slots. The face 3 and backing 4 sheet are laminated glass fibre, epoxy resin composite (the face sheet is perforated to allow sound waves to enter the chamber).
On top of the backing sheet as shown (i.e. radially outward of the backing sheet relative to the axis of rotation of the fan), polysulphide 4 is combed into ribs to provide some assembly compliance and damping; the polysulphide effectively acts as a compliant shim.
These all bonded assemblies form 90 degree arcs around the inner circumference of the casing. Although, typically, the partially circumferential rails are only produced in 45 degree arcs due to manufacturing constraints, and thus are often joined to form 90 degree arcs using thick plastic straps across the joint. The respective rails 1, 1′ are not directly connected to one another.
This results in 4 sub-assemblies per engine, collectively providing the fully circumferential assembly.
They are fitted into the fan casing, by removing the intake of the casing, by resting a rearward extension 5 of rail 1′ on a lip 6 formed as part of the fan casing geometry. Typically, there is an O-ring 6′ fitted into a groove on the lip 6 to provide assembly compliance and damping. Lip 6 provides a restraint against radial movement of the assembly. Lip 6 does not provide circumferential restraint.
The front region of the FAP is retained in place by multiple hook shaped metal brackets 7 arranged circumferentially around the fan casing. They are typically provided with bonded rubber cones 7′, again for compliance and damping. The brackets sit in a recess 8 on the main flange and are bolted in place using counter sink screws 9 so they are flush with the main flange surface.
Another method of fitting these FAPs is to radially bolt them to the casing, which means they can be taken out with the intake still assembled. However, this requires a large circumferential gap between sections which is undesirable from an aerodynamic perspective.
As will be appreciated, the FAP itself is a complex assembly requiring multiple manufacturing processes and materials. It is therefore labour intensive and costly to manufacture.
The assembly method also requires multiple separate components thus adding time and cost to install. The case itself must have provision for these components which require extra machining processes, adding cost and lengthening production time of the case.
FAPs are designed to attenuate noise and require the exposed face area to be maximised. However, the structures required by the conventional assemblies take up valuable axial space. For example, if a radial bolted solution is used, the bolt support structures themselves can also take up noise ‘real estate’, and can additionally affect aerodynamic performance due to the discontinuities (bolt heads) in the respective annulus line of the engine.